U.S. patent number 5,307,863 [Application Number 08/113,958] was granted by the patent office on 1994-05-03 for method for continuous casting of slab.
This patent grant is currently assigned to NKK Corporation. Invention is credited to Jun Kubota, Toshio Masaoka, Takashi Mori, Kazutaka Okimoto, Akira Shirayama.
United States Patent |
5,307,863 |
Kubota , et al. |
May 3, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Method for continuous casting of slab
Abstract
A method for continuous casting of a slab comprises feeding
molten steel into a mold through exit ports of an immersion nozzle
and controlling a stream of the molten steel by means of an
electromagnetic stirrer having a linearly shifting magnetic field.
The direction of the linearly shifting magnetic field is toward the
immersion nozzle, which is positioned at the center of the mold
from a pair of narrow sides of the mold. A first frequency control
step controls a frequency of a wave of the shifting magnetic field
to be higher than a threshold frequency, wherein the wave has a
period equal to the time during which the stream of the molten
steel poured from the immersion nozzle passes through an area to
which the linearly shifting magnetic field is introduced, said area
having an upper limit and a lower limit. A second control step
controls the frequency of the wave of the linearly shifting
magnetic field to be low enough such that the magnetic fluxes of
the linearly shifting magnetic field are of a density high enough
to apply a braking force to the molten steel.
Inventors: |
Kubota; Jun (Kawasaki,
JP), Shirayama; Akira (Kawasaki, JP),
Masaoka; Toshio (Kawasaki, JP), Okimoto; Kazutaka
(Kawasaki, JP), Mori; Takashi (Kawasaki,
JP) |
Assignee: |
NKK Corporation (Tokyo,
JP)
|
Family
ID: |
27169008 |
Appl.
No.: |
08/113,958 |
Filed: |
August 30, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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816608 |
Dec 31, 1991 |
|
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Current U.S.
Class: |
164/466;
164/468 |
Current CPC
Class: |
B22D
11/115 (20130101); B22D 11/186 (20130101); B22D
11/122 (20130101) |
Current International
Class: |
B22D
11/18 (20060101); B22D 11/115 (20060101); B22D
11/11 (20060101); B22D 11/12 (20060101); B22D
027/02 () |
Field of
Search: |
;164/466,468,502,504 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lin; Kuang Y.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Woodward
Parent Case Text
This application is a continuation of application Ser. No.
07/816,608, filed Dec. 31, 1991, now abandoned.
Claims
What is claimed is:
1. A method for continuous casting of a slab, comprising the steps
of:
feeding molten steel into a mold through exit ports of an immersion
nozzle, the mold having a pair of wide sides and a pair of narrow
sides, and the immersion nozzle being positioned at the center of
the mold from the pair of narrow sides;
controlling a stream of the molten steel by means of an
electromagnetic stirrer having a linearly shifting magnetic field,
a direction of the linearly shifting magnetic field being toward
the immersion nozzle, and distributions of magnetic fluxes of the
linearly shifting magnetic field being symmetrical relative to a
center line of the immersion nozzle;
a first control step of controlling a frequency of a wave of the
linearly shifting magnetic field to be higher than a threshold
frequency, said wave having said threshold frequency having a
period equal to the time during which the stream of the molten
steel fed into the mold from the immersion nozzle passes through a
field area to which the linearly shifting magnetic field is
introduced, said field area having an upper limit and a lower
limit; and
a second control step of controlling the frequency of the wave of
the linearly shifting magnetic field to be low enough such that the
magnetic fluxes of the linearly shifting magnetic field are of a
density high enough to apply a braking force to the molten
steel.
2. The method of claim 1, wherein said first control step comprises
controlling a frequency of an electric current for generating the
linearly shifting magnetic field to be a value such that when the
stream of the molten steel from the immersion nozzle falls outside
the lower limit of said field area, the value is determined by the
following formula:
where
F represents the value of frequency (Hz) of the electric current
for generating the linearly shifting magnetic field;
v represents average stream speed (m/sec.) of the molten steel fed
from the immersion nozzle when the stream of the molten steel
passes through the field area;
.theta. represents an angle (rad) formed by the stream of the
molten steel relative to a horizontal line when the stream of the
molten steel passes through the field area;
W represents a width (m) of the field area in a direction of a
height of the mold;
D represents a distance (m) from an upper end of the exit port of
the immersion nozzle to an upper limit of the field area, when the
upper end of the exit port of the immersion nozzle is located in
the field area; and
N represents a number of poles in the magnetic field generator.
3. The method of claim 1, wherein said first control step includes
controlling a frequency of electric current for generating the
linearly shifting magnetic field to be a value such that when the
stream of the molten steel fed from the immersion nozzle falls
within the upper limit and the lower limit of the field area, the
value is determined by the following formula:
where
F represents the value of frequency (Hz) of electric current for
generating the linearly shifting magnetic field;
v represents average stream speed (m/sec.) of the molten steel
poured from the immersion nozzle when the stream of the molten
steel passes through the field area;
.theta. represents an angle (rad) formed by the stream of the
molten steel relative to a horizontal line when the stream of the
molten steel passes through the field area;
A represents a width of a slab continuously cast; and
N represents a number of poles in the magnetic field generator.
4. The method of claim 1, wherein said first control step includes
controlling a frequency of an electric current to be greater than
or equal to a frequency F, the frequency F being determined by an
effective braking parameter E and an angle .alpha., the angle
.alpha. being formed by an axis of the exit port of the immersion
nozzle in a direction of the fed molten steel relative to a line
horizontal thereto and ranging from 60.degree. to 25.degree.
directed downwardly, said effective braking parameter E being
represented by the following formula:
where
A represents a width (m) of the mold for continuous casting of a
slab;
B represents a thickness (m) of the slab continuously cast;
C represents a speed (m/sec.) of the continuous casting;
S represents an effective area (m.sup.2) of the exit port of the
immersion nozzle;
N represents a number of poles in the magnetic field generator;
W represents a width (m) of the field area in a direction of a
height of the mold;
D represents a distance (m) from an upper end of the exit port of
the immersion nozzle to an upper limit of the field area, when the
upper end of the exit port of the immersion nozzle is located in
the field area; and
wherein said effective braking parameter E is represented by a
straight line connecting the point (E=0, F=0) and the point (E=5,
F=1.5) when the angle .alpha. ranges from 60.degree. to 35.degree.
both directed downwardly, the abscissa representing the effective
braking parameter E and the ordinate representing the frequency F
of electric current.
5. The method of claim 1, wherein said first control step includes
controlling a frequency of electric current for generating the
linearly shifting magnetic field to be greater than or equal to a
frequency F, the frequency F being determined by an effective
braking parameter E and an angle .alpha., the angle .alpha. being
formed by an axis of the exit port of the immersion nozzle in a
direction of the fed molten steel relative to a line horizontal
thereto and ranging over 25.degree. directed downwardly and below
15.degree. inclusive, directed upwardly, said effective braking
parameter E being represented by the following formula:
where
A represents a width (m) of the mold for continuous casting of a
slab;
B represents a thickness (m) of the slab continuously cast;
C represents a speed (m/sec.) of the continuous casting;
S represents an effective area (m.sup.2) of the exit port of the
immersion nozzle; and
N represents a number of poles in the magnetic field generator;
and
wherein said effective braking parameter E is represented by a
straight line connecting the points (E=0, F=0) and (E=5, F=1.3)
when the angle .alpha. ranges over 25.degree. directed downwardly
and below 15.degree. inclusive, directed upwardly, the abscissa
representing the effective braking parameter E and the ordinate
representing the frequency F of electric current.
6. The method of claim 1, wherein said first control step includes
controlling a frequency of electric current for generating the
linearly shifting magnetic field to be greater than or equal to a
frequency f, the frequency f being calculated by multiplying a
frequency F of electric current by an integer, and the frequency F
being determined by an effective braking parameter E and an angle
.alpha., the angle .alpha. being formed by an axis of the exit port
of the immersion nozzle in a direction of the fed molten steel
relative to a line horizontal thereto and ranging from 60.degree.
to 35.degree. directed downwardly, said effective braking parameter
E being represented by the following formula:
where
A represents a width (m) of the mold for continuous casting of a
slab;
B represents a thickness (m) of the slab continuously cast;
C represents a speed (m/sec.) of the continuous casting;
S represents an effective area (m.sup.2) of the exit port of the
immersion nozzle;
N represents a number of poles in the magnetic field generator;
W represents a width (m) of the field area in a direction of a
height of the mold;
D represents a distance (m) from an upper end of the exit port of
the immersion nozzle to an upper limit of the field area, when the
upper end of the exit port of the immersion nozzle is located in
the field area; and
wherein said effective braking parameter E is represented by a
straight line connecting the points (E=0, F=0) and (E=5, F=1.5)
when the angle .alpha. ranges from 60.degree. to 35.degree. both
directed upwardly, the abscissa representing the effective braking
parameter E and the ordinate representing the frequency F of
electric current.
7. The method of claim 1, wherein said first control step includes
controlling a frequency of electric current for generating the
linearly shifting magnetic field to be greater than or equal to
frequency f, the frequency f being calculated by multiplying
frequency F of electric current by an integer, and the frequency F
being determined by an effective braking parameter E and an angle
.alpha., the angle .alpha. being formed by an axis of the exit port
of the immersion nozzle in a direction of the fed molten steel
relative to a line horizontal thereto and ranging over 25.degree.
directed downwardly and below 15.degree. directed upwardly, said
effective braking parameter E being represented by the following
formula:
where
A represents a width (m) of the mold for continuous casting of a
slab;
B represents a thickness (m) of the slab continuously cast;
C represents a speed (m/sec.) of the continuous casting;
S represents an effective area (m.sup.2) of the exit port of the
immersion nozzle; and
N represents a number of poles in the magnetic field generator;
and
wherein said effective braking parameter E is represented by a
straight line connecting the points (E=0, F=0) and (E=5, F=1.3)
when the angle .alpha. ranges over 25.degree. directed downwardly
and below 15.degree. inclusive, directed upwardly, the abscissa
representing the effective braking parameter E and the ordinate
representing the frequency F of electric current.
8. The method of claim 1, wherein said second control step includes
controlling a frequency of an electric current for generating the
linearly shifting magnetic field so that the density of the
magnetic fluxes in the mold is at least 1200 gausses.
9. The method of claim 8, wherein the frequency of said electric
current is 2.8 Hz.
10. The method of claim 1, wherein said first control step includes
controlling a frequency of an electric current to be greater than
or equal to a frequency F, the frequency F being determined by an
effective braking parameter E and an angle .alpha., the angle
.alpha. being formed by an axis of the exit port of the immersion
nozzle in a direction of the fed molten steel relative to a line
horizontal thereto and ranging from 35.degree. to 25.degree.
directed downwardly, said effective braking parameter E being
represented by the following formula:
where
A represents a width (m) of the mold for continuous casting of a
slab;
B represents a thickness (m) of the slab continuously cast;
C represents a speed (m/sec.) of the continuous casting;
S represents an effective area (m.sup.2) of the exit port of the
immersion nozzle;
N represents a number of poles in the magnetic field generator;
W represents a width (m) of the field area in a direction of a
height of the mold;
D represents a distance (m) from an upper end of the exit port of
the immersion nozzle to an upper limit of the field area, when the
upper end of the exit port of the immersion nozzle is located in
the field area; and
wherein said effective braking parameter E is represented by a
straight line connecting the point (E=0, F=0) and the point (E=5,
F=1.4) when the angle .alpha. ranges from 35.degree. to 25.degree.
directed downwardly, the abscissa representing the effective
braking parameter E and the ordinate representing electric current
frequency.
11. The method of claim 1, wherein said first control step includes
controlling a frequency of electric current for generating the
linearly shifting magnetic field to be greater than or equal to a
frequency f, the frequency f being calculated by multiplying a
frequency F of electric current by an integer, and the frequency F
being determined by an effective braking parameter E and an angle
.alpha., the angle .alpha. being formed by an axis of the exit port
of the immersion nozzle in a direction of the fed molten steel
relative to a line horizontal thereto and ranging from 60.degree.
to 35.degree. directed downwardly, said effective braking parameter
E being represented by the following formula:
where
A represents a width (m) of the mold for continuous casting of a
slab;
B represents a thickness (m) of the slab continuously cast;
C represents a speed (m/sec.) of the continuous casting;
S represents an effective area (m.sup.2) of the exit port of the
immersion nozzle;
N represents a number of poles in the magnetic field generator;
W represents a width (m) of the field area in a direction of a
height of the mold;
D represents a distance (m) from an upper end of the exit port of
the immersion nozzle to an upper limit of the field area, when the
upper end of the exit port of the immersion nozzle is located in
the field area; and
wherein said effective braking parameter E is represented by a
straight line connecting the point (E=0, F=0) and the point (E=5,
F=1.5) when the angle a ranges from 60.degree. directed downwardly
to 35.degree. inclusive, directed upwardly, the abscissa
representing the effective braking parameter E and the ordinate
representing the frequency F of the electric current.
Description
BACKGROUND OF THE INVENTION
1. Field of the Industrial Application
The present invention relates to a method for continuous casting of
a steel slab, and more particularly to a method for continuous
casting of a slab wherein a wave of a molten steel surface is
depressed by introducing an electro magnetic force to the molten
steel in a mold.
2. Description of the Related Art
Molten steel is usually poured from a tundish into a mold through
an immersion nozzle to prevent the molten steel from being
oxidized. The immersion nozzle prevents the molten steel from being
exposed to the air. The immersion nozzle for continuous casting of
a slab has a pair of exit ports having openings at its lower end.
Molten steel is poured into a mold through the exit ports of the
immersion nozzle positioned at the center of the mold toward the
circumference inside the mold.
It has been an object in recent years in the field of continuous
casting of steel to increase casting speed, namely, the speed of
pouring molten steel into a mold for increasing a productivity of a
continuous casting machine. However, when the casting speed is
increased to more than 1.5 m/min, molten steel in the mold is
violently disturbed. Various waves of the molten steel of
wavelengths from several meters down to several centimeters are
generated on the surface of the molten steel, with a portion of the
immersion nozzle being fulcrum, whereby the wave height of the
molten steel becomes large. Mold powder is also entangled in the
molten steel by such a wave of the molten steel surface. The mold
powder entangled in the molten steel and non-metallic inclusions
produced during a refining process are prevented from rising up to
the surface or the molten steel by the violent disturbance of the
molten steel in the mold. As a result, those inclusions are hard to
remove from the molten steel in the mold. The inclusions entangled
in a slab will appear as surface defects and inner defects of a
product that has passed through a final processing. Those surface
defects and inner defects of a product greatly the lower quality of
the product.
As a prior art to prevent such inclusions from being entangled in a
slab, a method for electromagnetically stirring molten steel in a
mold, which is disclosed in Japanese Examined Patent Publication
No. 10305/89, can be pointed out. In the prior art, an
electromagnetic stirrer is placed near the meniscus on a wide side
of a mold in a continuous casting apparatus. An electromagnetic
inducing force is applied to the molten steel in such a direction
so as to force back the molten steel along a direction of a width
of the mold from a narrow side of the mold toward the immersion
nozzle by means of the electromagnetic stirrer. A flow speed of the
molten steel poured into the mold from the immersion nozzle is
decreased. Owing to the decrease of the flow speed, the wave motion
of the molten steel surface in the mold is decreased and a
disturbance of the molten steel therein is depressed.
The magnetic field generator used in the prior art is of a linearly
shifting magnetic field type. Therefore, an appropriate value and a
frequency of electric current should be determined. The frequency
has been determined as follows:
The Lorentz force acting on a poured stream of the molten steel
should be enhanced to elevate the damping ratio of the flow speed
of the poured molten steel. To enhance the Lorentz force, a
relative speed of the poured stream of molten steel to a magnetic
flux from the narrow side of the mold toward the immersion nozzle
should be increased. Accordingly, a shifting speed of the magnetic
flux, that is, a frequency of the magnetic flux should be
increased. However, when the frequency of the magnetic flux is
increased, the magnetic permeability of stainless steel and mold
copper plate composing a frame of the mold is lowered, and the
magnetic permability of the molten steel is also lowered. As a
result, the density of the magnetic flux acting effectively on the
poured stream of the molten steel from the immersion nozzle is
decreased. A frequency of 0.5 Hz has customarily been used as the
appropriate frequency satisfying a condition of both Lorentz force
and the magnetic permeability.
FIG. 1 is a graphical representation showing the magnitude of a
wave of a molten steel surface in a mold, when the value of
electric current in a magnetic field generator is varied under the
condition of electric current frequency of 0.5 Hz in the magnetic
field generator. The direction of shift of a magnetic field is the
direction from the narrow side of the mold toward the immersion
nozzle. The magnitude of the wave is represented with an average
value of the amplitude of wave of a the molten steel surface, which
are obtained by measuring the amplitude of the wave of the molten
steel for ten minutes, at positions 40 mm away from the narrow side
of the mold and 40 mm away from the wide side of the mold. As shown
in FIG. 2, the wave motions are substantially composed of a short
period wave 30 having a period of about 1 to 2 seconds and a long
period wave 31 having a period of about 10 to 15 seconds. The
amplitude of the wave of the molten steel is a wave height
difference 32 between two wave heights. One is a wave height
showing the maximum height of the short period wave at a moment
closest to a moment when the long period wave shows the maximum
height, and the other is a height of wave showing the minimum
height of the short period wave at a moment when the long period
wave shows the minimum height. Lines A, B, C and D in FIG. 1 were
carried out under the following conditions.
In line A, a mold had a width of 850 mm. An immersion nozzle had
square openings each directed downwardly at 35.degree. relative to
a horizontal line. A casting speed of molten steel was 1.6 m/min.
In line B, a mold had a width of 1050 mm. An immersion nozzle had
square openings each directed downwardly at 35.degree. relative to
a horizontal line. A casting speed of molten steel was 1.8 m/min.
In line C, a mold had a width of 1250 mm. An immersion nozzle had
square openings each directed downwardly at 45.degree. relative to
a horizontal line. A casting speed of molten steel was 2.3 m/min.
In line D, a mold had a width of 1350 mm. An immersion nozzle had
square openings each directed downwardly at 45.degree. relative to
a horizontal line.
A casting speed of molten steel was 2.0 m/min. In any of the cases
of the lines A, B, C and D, a frequency in a magnetic field
generator was 0.5 Hz.
Under the conditions of A and B that the casting speed of molten
steel is comparatively small and the width of the mold is small, as
electric current in the magnetic field generator is increased, the
effect of depressing the wave of the molten steel surface
increases. But, under the conditions of C and D that the casting
speed of molten steel is comparatively large and the width of the
mold is large, when electric current in the magnetic field
generator is excessively increased, the effect of depressing the
wave of the molten steel decreases, which promotes the increase of
the wave motions, contrary the goals of the technique.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for
continuous casting of a slab wherein a wave of molten steel in a
mold can be depressed under a flexible control condition of
operation.
To attain the above-mentioned object, the present invention
provides a method for continuous casting of a slab, comprising the
steps of:
feeding molten steel into a mold through exit ports of an immersion
nozzle, the mold having a pair of wide sides and a pair of narrow
sides, and the immersion nozzle being positioned at the center of
the mold from the pair of narrow sides;
controlling a stream of the molten steel by use of an
electromagnetic stirrer having a linearly shifting magnetic field,
a direction of the linearly shifting magnetic field being toward
the immersion nozzle and distributions of magnetic fluxes of said
linearly shifting magnetic field being symmetrical with respect to
a center line of the immersion nozzle;
a first control step of controlling a frequency of said shifting
magnetic flux to be higher frequency than a specific frequency with
which the cycle time of the shifting magnetic flux of said shifting
magnetic field is equal to the travelling time of said molten steel
stream within said shifting magnetic field;
a second control step of controlling a frequency of said shifting
magnetic flux to be a lower frequency with which the flux density
of said shifting magnetic field in the mold cavity is strong enough
to interact with said molten steel stream to give a braking effect
to said molten steel stream.
The above objects and other objects and advantages of the present
invention will become apparent from the following detailed
description, taken in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical representation showing a magnitude of a wave
of a molten steel surface adjacent to the narrow side of a mold
when a frequency of electric current in a magnetic field generator
is 0.5 Hz;
FIG. 2 (A) and (B) are graphical representations explaining the
definition of an amplitude of the wave of the molten steel
surface;
FIG. 3 is a schematic illustration showing a stream of the molten
steel poured into the mold from an immersion nozzle of the present
invention;
FIG. 4 is a graphical representation showing the relationship
between frequency of an electric current in the magnetic field
generator and an average maximum value of the magnetic fluxes per
hour, which is obtained by calculation,
FIG. 5 is a vertical sectional view illustrating an apparatus for
controlling a molten steel surface used in the method for
continuous casting of the present invention;
FIG. 6 is a wiring diagram showing a coil of the magnetic field
generator seen from the upper side of the mold;
FIG. 7 is a graphical representation showing the results of an
operation of continuous casting which depresses a wave of the
molten steel surface adjacent to the narrow side of the mold, the
operation being carried out under the condition of a large width of
the mold and a comparatively large casting speed of molten
steel;
FIG. 8 is a graphical representation showing the results of an
operation of continuous casting which depresses a wave of the
molten steel surface adjacent to the narrow side of the mold, the
operation being carried out under the condition of a large width of
the mold and a comparatively large casting speed of molten
steel;
FIG. 9 is a graphical representation showing the results of an
operation of continuous casting which depresses wave of the molten
steel surface adjacent to the narrow side of the mold, the
operation being carried out under the condition of a large width of
the mold and a comparatively large casting speed of molten
steel;
FIG. 10 is a graphical representation showing the results of an
operation of continuous casting which depresses a wave the molten
steel surface adjacent to the narrow side of the mold, the
operation being carried out under the condition of a large width of
the mold and a comparatively large casting speed of molten
steel;
FIG. 11 is a graphical representation showing the results of FIGS.
7 to 10, the frequency of electric current being represented by the
abscissa and the wave adjacent to the narrow side of the mold by
the ordinate;
FIG. 12 is a graphical representation showing a change of the
effect of depressing the wave of the molten steel surface adjacent
to the narrow side of the mold when the value of electric current
in the magnetic field generator is varied;
FIG. 13 is a graphical representation representing the lower limit
of a frequency of electric current for depressing the wave of the
molten steel surface with an effective braking parameter and an
angle of the axis of the exit port of the immersion nozzle in the
direction of poured molten steel; and
FIG. 14 is a graphical representation showing a straight line
indicating a lower limit of a frequency of electric current for
depressing the wave of the molten steel surface and a straight line
indicating a frequency of electric current obtained by multiplying
the above frequency by an integer .
DESCRIPTION OF PREFERRED EMBODIMENT
The magnetic field generator of the present invention is of a
linearly shifting magnetic field type. A magnetic flux shifts from
the narrow side of a mold toward an immersion nozzle in a direction
crossing at a right angles to the direction of withdrawal a slab.
Alternatively the magnetic flux shifts from the narrow side of the
mold toward the immersion nozzle at a certain angle to the
direction crossing at a right angle the direction of withdrawal of
the slab. That is to say, the magnetic flux is applied in an
adverse direction against the stream of the molten steel poured
from the immersion nozzle. Accordingly, a density of the magnetic
flux at a certain point inside the mold varies periodically.
Therefore, the stream of the molten steel poured from the immersion
nozzle does not always cross a magnetic flux having a constant
density in terms of time. There occurs a difference in the total
amount of electromagnetic forces received by the stream of the
molten steel until the molten steel has passed through an area to
which the linearly shifting magnetic field is introduced, depending
on a difference in moments when the molten steel is poured from the
immersion nozzle.
The present inventors have found the following:
Firstly, a period of time, necessary for a certain fragment of the
stream of the molten steel poured from the immersion nozzle to pass
through an area to which the linearly shifting magnetic field is
introduced, is determined by a width of the mold, an amount of the
molten steel poured from the immersion nozzle, an angle of
discharge of molten steel from the immersion nozzle, a depth of
exit ports of the immersion nozzle immersed into the molten steel
and a frequency of electric current in the magnetic field
generator. The amount of the molten steel is determined by the
width of the mold and a casting speed.
Secondly, times of crossings of magnetic fluxes with stream of
molten steel while the stream of the molten steel poured from the
mold are passing through an area to which a linearly shifting
magnetic field is introduced, are determined by a width of a mold,
an average amount of molten steel poured from the immersion nozzle
which is determined by the width of the mold and a casting speed,
an angle of the molten steel poured from the immersion nozzle, a
depth of exit ports of the immersion nozzle immersed into the
molten steel and a frequency of electric current in the magnetic
field generator.
Thirdly, flow velocity of the molten steel stream at the exit of
said shifting magnetic field is reduced by the braking effect of
electromagnetic forces obtained from the interaction with said
shifting magnetic flux. It is possible to keep uniform the velocity
over all fragments of the molten steel stream, if every fragment of
the molten steel stream receives almost the same amount of
electromagnetic force in the magnetic field.
Variation of the flow velocity and the increase of wave motion in
the molten steel surface take place, when a fragment of the molten
steel stream does not receive a same amount of, or receives a
lesser amount of, electromagnetic force in the magnetic field.
In order to make sure that any fragment of the molten steel stream
has the same amount of accumulated electromagnetic force in the
magnetic field, the stream has to receive a minimum one cycle of,
and preferably plural cycles of, the shifting magnetic flux in the
magnetic field. Two methods are conceivable for accomplishing
this.
A first method is a method wherein molten steel poured from the
immersion nozzle passes, through the area, to which the linearly
shifting magnetic field is introduced with a passing time which is
as long as possible. A speed of the stream of the molten steel
poured from the immersion nozzle is decreased by decreasing a
casting speed. Alternatively, the stream of the molten steel poured
from the immersion nozzle is caused to flow in a direction parallel
to the direction of shift of the magnetic flux in the area to which
the linearly shifting magnetic field is introduced, by making
smaller an angle of the molten steel poured from the immersion
nozzle with regard to a horizontal line. However, when the casting
speed is decreased, the production efficiency of a continuous
casting machine is lowered. When the angle of the molten steel
poured from the immersion nozzle is decreased, mold powder in the
stream of the molten steel is entangled, which gives rise to the
entanglement of inclusions in a slab. Therefore, this first method
is not advantageous.
The second method is a method wherein the frequency of electric
current of the magnetic field generator is selected and a shifting
speed of magnetic fluxes of the linearly shifting magnetic field is
controlled. The frequency of electric current is set at a necessary
minimum frequency or more so that any of the fragments of the
stream of the molten steel can cross the moving magnetic flux at
least once while the fragment of the molten steel poured from the
immersion nozzle is passing through the area to which the linearly
shifting magnetic field is introduced. That is to say, since any of
the fragments of molten steel poured from the immersion nozzle
undergoes at least once a braking force of the density of the
magnetic flux of one cycle of the linearly shifting magnetic field
during its passing through the area to which the linearly shifting
magnetic field is introduced, there occurs no unevenness of degree
of the introduction of the magnetic field to the molten steel,
i.e., there is no unbalance wherein some parts of the molten steel
are braked while others are not. If the selected frequency is a
necessary minimum frequency or a frequency determined by
multiplying the minimum frequency by an integer, any of the
fragments of molten steel undergo the braking force equally, and
the wave of the molten steel surface in the mold is further
decreased.
According to this second method, since there is no direct influence
on the casting speed and the angle of the molten steel poured from
the immersion nozzle, the wave of the molten steel on surface can
be decreased. However, when the frequency of electric current in
the magnetic field generator is increased, the magnetic
permeability is lowered, which lowers the density of the magnetic
flux acting effectively on the stream of the molten steel poured
from the immersion nozzle. Accordingly, this frequency is desired
to be the minimum necessary frequency found by using the method
described below or the frequency produced by multiplying the
minimum frequency by an integer. For example, the frequency
multiplied by an integer becomes a frequency multiplied by two or
three. Since the braking force with which the shifting magnetic
field acts on the fragments of the molten steel poured from the
immersion nozzle increases in proportion to the product of the
square of the magnetic flux and the frequency, it is effective to
select a frequency multiplied by an integer which maximizes the
product.
The minimum frequency of electric current necessary in the second
method is found as follows:
An interval of time P [sec] at which the magnetic flux is shifted,
passes periodically in the magnetic field generator, and is
represented by the formula (1):
where N is a number of poles in the magnetic field generator and F
is a frequency of electric current in the magnetic field generator
[Hz].
FIG. 3 is a schematic illustration showing a stream of molten steel
poured from the immersion nozzle of the present invention. As shown
in FIG. 3, the molten steel poured from the exit ports 29 of the
immersion nozzle enters the area to which the linearly shifting
magnetic field is introduced, reaches the lower end 34 of the area,
and exits the area. The period of time from the entry of the molten
steel into the area to the exiting of the molten steel from the
area, that is, an effective braking period of time T[sec.] is
represented by the formula (2).
where
V is an average speed of the stream of the molten steel [m/sec.] at
which the stream of the molten steel poured from the immersion
nozzle passes through the area. The area to which the linearly
shifting magnetic field is introduced is an area which has a
density of the magnetic flux of 1/2 of the maximum value as an
average value of the magnetic flux per hour, which is measured at
the center of the mold in the direction of the thickness of the
mold;
.theta. is an angle[rad] formed by the stream of the molten steel
poured from the exit ports of the immersion nozzle relative to a
horizontal line when the stream of the molten steel passes through
the area to which the linearly shifting magnetic field is
introduced;
W is a width[m] of the area to which the linearly shifting magnetic
field is introduced in the direction of the height of the mold;
and
D is a distance[m] from the upper end of the exit port of the
immersion nozzle to the upper end of the area to which the linearly
shifting magnetic field is introduced, when the end of the exit
port of the immersion nozzle is located in the area to which the
linearly shifting magnetic field is introduced, D being equal to 0
[m] when the end of the exit port of the immersion nozzle is out of
the introduced area.
On the other hand, when a downwardly directed angle .alpha. of the
exit port of the immersion nozzle is small or an angle formed by
the direction of the stream of the molten steel poured from the
immersion nozzle and the direction of the shifting of the magnetyic
flux is small, the stream reaches a solid shell adjacent to the
narrow sides of the mold before the stream of the molten steel goes
out of the upper limit or the lower limit of the linearly shifting
magnetic field. The time which the stream of the molten steel takes
from existing the exit port of the immersion nozzle to arrival at
the solid shell adjacent to the narrow side of the mold is an
effective braking time T[sec.]. The time is represented by the
following formula(3):
where A is a width of cast slab.
It is very difficult to actually measure the values of V and
.theta. in an operation of a continuous casting machine. Therefore,
the present inventors reproduced an actual casting by using a water
model to measure V and .theta.. However, a braking effect by the
magnetic field generator was not added to the values of V and
.theta..
From the formulae (1) (2) and (3), by making P=T, there is
determined a minimum frequency necessary to achieve uniformity of
the total amount of magnetic fluxes which any of the fragments of
molten steel poured from the immersion nozzle crosses during its
passing through the area to which the linearly shifting magnetic
field is introduced.
The minimum frequency of electric current is represented by the
following formula (4) in a case where the stream of the molten
steel poured from immersion nozzle goes out of the lower limit of
the linearly shifting magnetic field:
The minimum frequency of electric current is represented by the
following formula (5) in a case where the stream of the molten
steel poured from immersion nozzle is in the range of between the
lower limit and the upper limit of the linearly shifting magnetic
field:
In FIG. 3, symbols in the formula (4) and (5) are explained. Molten
steel is poured into a mold from exit ports 29 of immersion nozzle
8. The molten steel poured from the exit ports of the immersion
nozzle 8 passes through an area to which a linearly shifting
magnetic field is introduced at an average flow speed 27 (V) and at
an angle 26 (.theta.) to the horizontal line. Reference numeral 24
denotes a width of a magnetic field generator in the direction of a
height of a coil.
A width 23 (W) of the linearly shifting magnetic field in the
direction of a height of the mold in the area to which the linearly
shifting magnetic field is introduced is in between the upper end
33 and the lower end 34 of the introduced area. In the case where
the upper end of the exit port of the immersion nozzle is located
in the area to which the linearly shifting magnetic field is
introduced, the shifting magnetic field does not act effectively on
the stream of the molten steel in the area of a distance 25 (D)
from the upper end of the exit port of the immersion nozzle to the
lower end 34 of the area to which the linearly shifting magnetic
field is introduced. The molten steel poured into the mold having
the upper end 20 and the lower end 22 has a molten steel surface
21.
FIG. 4 is a graphical representation showing the relationship
between the frequency of electric current in the magnetic field
generator and the maximum value of average magnetic fluxes per hour
in the mold, which was measured in a continuous casting machine.
When the frequency of electric current is increased, a magnetic
permeability of stainless steel plate and copper plate composing a
frame of the mold is lowered, which lowers the densities of the
magnetic fluxes. The densities of the magnetic fluxes in the mold
of continuous casting machines are not always equal to those in
FIG. 4 because of differences of structures and performances of
individual apparatuses. According to the test conducted by the
present inventors, in order to effectively brake a flow speed of
the molten steel poured from an immersion nozzle, it is desirable
that densities of magnetic fluxes in the mold be at least 1200
gauss. In the case of FIG. 4, a frequency of electric current of
2.8 Hz or less is selected, and the shifting speed of the linearly
shifting magnetic filed is controlled.
However, since the values of the average flow speed of the molten
steel and the angle .theta. cannot be measured in an actual
operation of a continuous casting, there is inconvenience in that a
necessary minimum frequency or a frequency which is calculated by
multiplying the minimum frequency by an integer are not immediately
obtained. The present inventors have found a way of solving the
inconvenience.
The results of the test conducted by the above-mentioned water
model was compared with those conducted by a continuous casting
machine, using an effective braking parameter E. The effective
braking parameter E is calculated according to a width A[m] of a
mold for continuous casting, a thickness B[m] of casting, a casting
speed C[m/sec.] and an effective area S[m.sup.2 ] of the exit port
of the immersion nozzle.
The test by the continuous casting was carried out under the
following conditions:
a width of the slab cast was 0.7 to 2.6 m; the thickness of the
slab cast was 0.1 to 0.3 m; the casting speed was 0.6 to 5.0
m/min.; the angle of poured molten steel from an immersion nozzle
ranged from 60.degree. directed downwardly to 15.degree. directed
upwardly; and the capacity of continuous casting machine per strand
was 15 ton/min.
The water model test was carried out corresponding to the
conditions of the above test by continuous casting.
Using the effective braking parameter E and the angle .alpha. of
the molten steel poured from the exit port of the immersion nozzle,
the minimum frequency F of electric current necessary for
controlling the wave of the molten steel in the mold is represented
as seen in FIG. 13. In FIG. 13, .alpha. is an angle formed by an
axis of the exit port of the immersion nozzle and a horizontal
line. The frequency calculated by multiplying the minimum frequency
by an integer is represented shown as in FIG. 14.
An effective braking parameter E is determined responsive to the
angle .alpha. formed by an axis of the exit port of the immersion
nozzle and the horizontal line. The parameter E is represented by
the following formula (6) in the case where the angle .alpha. is
within the range of 60.degree. to 25.degree. directed
downwardly:
The parameter E is represented by the following formula (7) in the
case where the angle .alpha. is within the range of over 25.degree.
directed downwardly and below 15.degree. directed upwardly:
The formulas (6) and (7) are calculated according to a width A[m]
of a mold for continuous casting, a thickness B[m] of casting, a
casting speed C[m/sec.] and an effective area S[m.sup.2 ] of the
exit port of the immersion nozzle. The area S [m.sup.2 ] is a
section area crossing perpendicularly to the axis of the exit port
of the immersion nozzle and the shape of the section area can be
such as a circle, an elipse, a square, a rectangle and an
egg-shape.
In FIG. 13, each of the straight lines is drawn corresponding to
the respective angles .alpha. the exit port. Straight line(a) shows
a case of the angle .alpha. being in the range from 60.degree. to
35.degree. both directed downwardly, straight line(b) a case of the
angle .alpha. being in the range from over 35.degree. to 25.degree.
directed downwardly, and straight line(c) a case of the angle
.alpha. being in the range from over 25.degree. directed downwardly
and 15.degree. inclusive, directed upwardly. The straight line(a)
connects points (E=0, F=0) and (E=5, F=1.5), the straight line(b)
points (E=0, F=0) and (E=5, F=1.4) and the straight line(c) points
(E=0, F=0) and (E=5, F=1.3).
EXAMPLE
An example of the present invention will now be described with
specific reference to the appended drawings.
FIG. 5 is a vertical sectional view illustrating a molten steel
surface controller used in the method for continuous casting of
steel of the present invention. A tundish 2 is mounted above a mold
10 for continuous casting, and molten steel is fed from a ladle
(not shown) to the tundish 2. A inside wall of the tundish is lined
with refractory 3, and an outside of the tundish is covered with a
steel shell 4. A sliding nozzle 5 is placed at a bottom of the
tundish 2. The sliding nozzle 5 has an immovable plate 6 fixed to
the steel shell 4 and a sliding plate 7 sliding relative to the
immovable plate 6. The nozzle 5 is opened and closed by sliding the
sliding plate 7.
An immersion nozzle 8 is fixed to the lower side face of the
sliding plate 7. A lower end portion of the immersion nozzle 8 is
immersed in a molten steel 1 already poured into the mold 10. The
molten steel 1 is poured into the mold 10 through a pair of exit
ports 9 placed symetrically on both left and right sides thereof. A
molten steel surface sensor 14 is arranged facing the surface of
molten steel in the mold to detect positions of the molten steel
surface and changes of the positions of the molten steel surface.
The molten steel surface sensor 14 is connected to an input side of
a monitor in a control device 16 for controlling a sliding nozzle
opening angle. Independently from the molten steel surface sensor
14, two molten steel surface sensors 17 are positioned on the
narrow sides of the mold, each of the sensors being on each of the
both narrow sides of the mold. This molten steel surface sensor 17
is not connected to the control device 16. The molten steel surface
sensor 17 monitors the effect of depressing the movement of the
wave of the molten steel surface generated by the magnetic field
generator of the present invention. The magnetic field generator 18
is placed behind copper plates of both wide sides of the mold.
Table 1 shows a composition of steel provided for the casting of
the Examples of the present invention.
Table 2 shows operation conditions of the casting of the Examples
of the present invention.
Table 3 shows a specification of the magnetic field generator used
in the casting of the Example of the present invention.
TABLE 1 ______________________________________ Composi- Soluble
tion C Si Mn S P Al ______________________________________ Range
0.03.about. 0.04 0.10.about. 0.025 0.25 0.030.about. (wt. %) 0.08
or 0.25 or or 0.070 less less less
______________________________________
TABLE 2 ______________________________________ Width of Mold 1550
mm; 950 mm Thickness of Cast Slab 230 mm Casting Speed 2.0 m/min.;
1.6 m/min. Flow Rate of Ar gas Blown 10.0 N l/min into Immersion
Nozzle Immersion Nozzle Inside Diameter: 90 mm; Exit Port:
Square-Shaped; and Angle of Exit Port: 45.degree. directed
downwardly Temperature of Molten 1545.about.1565.degree. C. Steel
in Tundish Immersion Depth of 270 mm above Molten Steel Sur- Exit
Port of Immersion face (Position of Upper End Nozzle Limit of
Immersion Nozzle) ______________________________________
TABLE 3 ______________________________________ Magnetic Field
Linearly Shifting Magnetic Field Capacity 2000KVA/strand
(Three-phase Alternating Current) Voltage Max. 430 V/strand
Electric Current Max. 2700 A/strand Frequency of Electric
0.about.2.6 Hz Current Number of Poles 2 Maximum Density of 0.2
Tesra Magnetic Flux B W 0.48 m
______________________________________
The maximum density B of the magnetic flux shown in Table 3 is an
average density of magnetic flux per hour at a point where an
average density of magnetic flux per hour, which is measured at the
center of the mold in the direction of the thickness thereof, shows
a maximum value. W in Table 3 is a width of an area in the
direction of the height of the mold, which has an average density
of magnetic flux per hour of 1/2 of the maximum value of the
density of magnetic flux with a position as the center, which shows
the maximum value of the average density of magnetic flux per hour,
and which is measured at the center of the mold in the direction of
the thickness thereof.
FIG. 6 is a wiring diagram showing a coil in the magnetic field
generator used in the present invention.
EXAMPLE-1
Continuous casting of a slab was carried out by controlling the
surface of molten steel in the mold by the magnetic field generator
as shown in Table 3. The casting conditions are as shown in Table
2.
Firstly, an average flow V of the molten steel and an angle .theta.
under the casting conditions as shown in Table 2 were measured in a
water model test wherein a model of a mold scaled down to 1/3 of an
actual mold was used. Measured values were converted in calculation
to those of a scale of an actual apparatus operation. The values of
V=1.15 m/sec and .theta.=0.70 were obtained. A period of time [sec]
necessary for a minute stream of the molten steel poured from the
immersion nozzle to enter an area to which a linearly shifting
magnetic field is introduced, and to exit the introduced area is
calculated by substituting the said values of V and .theta. for the
formula(3), and the time T=0.56(sec.) is obtained.
Accordingly, to effectively depress a wave of the molten steel
surface when a casting speed is comparatively large and a width of
a mold is large, a time period P[sec.] for which the magnetic
fluxes pass periodically through the area, to which a linearly
shifting magnetic field is introduced, is determined at 0.56 sec.
or less. A frequency F of electric current in the magnetic field
generator when the time period P[sec.] is determined to be 0.56
sec. or less is calculated by the formula(3) to be 0.89 (Hz) or
more.
By using the above-mentioned results an operation of continuous
casting wherein the casting speed was comparatively large and the
width of the mold was large was carried out by depressing the wave
of the molten steel surface. The results of the operation are shown
in FIGS. 7.
The abscissa in FIG. 7 represents time. The time lapses from the
right to the left on the graph. The ordinate represents height of
the molten steel surface adjacent to the narrow side of the mold
which is measured by the molten steel surface sensor 17. The
operation conditions for the results in FIG. 7 is listed in Table
2. FIG. 7 shows the results of comparison in the case where the
magnetic field generator was not used. Since the magnetic field
generator was not used, the surface molten steel adjacent to the
narrow side of the mold was greatly fluctuated. To depress this
fluctuation of the surface molten steel, the magnetic field
generator is driven.
FIG. 8 shows comparison wherein the magnetic field generator was
driven with a frequency of electric current of 0.5 Hz and with a
value of electric current of 1080 A. The frequency of electric
current of 0.5 Hz is lower than the lower limit of the frequency of
electric current of 0.89 Hz which effectively depresses the wave of
the molten steel surface in the mold the casting speed is
comparatively large and the width of the mold is large. That is,
the necessary condition for the lower limit of the frequency of
electric current under the operation condition as shown in Table 2
is not satisfied. Actually, as shown in FIG. 8, there is
substantially no effect of depressing the wave of the molten steel
surface adjacent to the narrow side of the mold. On the contrary,
the wave of the molten steel surface is accelerated.
FIG. 9 shows an example wherein the magnetic field generator is
driven with a frequency of electric current of 1.0 Hz and with the
value of 1080 A. The frequency of electric current of 1.0 Hz is
higher than the lower limit of the frequency of electric current of
0.89 Hz, which effectively depresses the wave of the molten steel
surface when that the casting speed is comparatively large and the
width of the mold is large. That is, the necessary condition for
the lower limit of the frequency of electric current under the
operation condition as shown in Table 2 is satisfied. It is well
understood that the effect of depressing the wave of the molten
steel surface adjacent to the narrow side of the mold is actually
great as shown in FIG. 9.
FIG. 10 shows an example wherein the magnetic field generator was
driven with a frequency of electric current of 2.0 Hz and with a
value of electric current of 1080 A. The frequency of electric
current of 2.0 Hz is higher than the lower limit of the frequency
of electric current of 0.89 Hz which effectively depresses the wave
of the molten steel surface when the casting speed is comparatively
large and the width of the mold is large. That is, the necessary
condition for the lower limit of the frequency of electric current
under the operation condition as shown in Table 2 is satisfied. It
is also well understood that the effect of depressing the wave of
the molten steel surface adjacent to the narrow side of the mold is
actually great as shown in FIG. 10.
FIG. 11 shows the relationship of the wave of the molten steel
surface adjacent to the narrow side of the mold to the frequency of
electric current, which is obtained by summing up the results as
shown in FIGS. 7 to 10. The abscissa represents the frequency of
electric current and the ordinate the wave of the molten steel
surface. The wave of the molten steel surface is sufficiently
depressed by use of a frequency higher than the lower limit of the
frequency of electric current of 0.89 Hz for effectively depressing
the wave of the molten steel surface.
EXAMPLE 2
FIG. 12 shows the relationship between the value of electric
current in the magnetic field generator and the magnitude of the
wave of the molten steel surface adjacent to the narrow side of the
mold. The casting conditions are those shown in Table 2. Lines A,
B, C and D in FIG. 12 were carried out under the following
conditions:
For lines A and B, a width of a mold was 950 mm. An immersion
nozzle had square openings directed downwardly at 45.degree. to the
horizontal line. A casting speed was 1.6 m/min. In line A, a
frequency of electric current was 0.5 Hz. In line B, a frequency of
electric current was 1.0 Hz. In lines C and D, a width of a mold
was 1550 mm. An immersion nozzle had square openings directed
downwardly at 45.degree. to the horizontal line. A casting speed
was 2.0 m/min. In line C, a frequency of electric current was 0.5
Hz. In line D, a frequency of electric current was 1.0 Hz.
In FIG. 12, lines A and B show the case where a casting speed was
comparatively small and a width of a mold was small. When the
frequencies of electric current were 0.5 Hz and 1.0 Hz, the effect
of depressing the wave of the molten steel surface adjacent to the
narrow side of the mold was obtained in correspondence with each of
the values of electric current. V was 0.67 m/sec, .theta. was 0.43
rad, and W was 0.48 under the casting conditions of A and B. The
lower limit of the frequency of electric current found by the
formula (3) was 0.43 Hz. Since the magnetic field was generated by
the lower limit of the frequency of electric current of 0.43 Hz or
more, the effect of depressing the wave of the molten steel surface
was sufficiently produced. An effective braking parameter E was
1.2.
In FIG. 12, lines C and D show the case where the casting speed is
comparatively large and the width of the mold is large. Under the
casting conditions of the lines C and D, V is 1.15 m/sec, .theta.
0.66 rad, and W 0.48 m. The lower limit of the frequency of
electric current is 0.89 Hz. The effective braking parameter is
2.6. The case of the line C is the case where the frequency of
electric current is 0.5 Hz, which is lower than the lower limit of
the frequency of electric current F of 0.89. In this case, when the
value of electric current was increased, the wave of the molten
steel surface is accelerated. The case of the line D is a case
where the frequency of electric current is 1.0 Hz, which is higher
than the lower limit of the frequency of electric current F of
0.89. The effect of depressing the wave of the molten steel surface
is obtained in correspondence with each of the values of electric
current.
A lower limit of a frequency of electric current for depressing
wave of the molten steel surface in the mold is shown in FIG. 13.
In the case of FIG. 13, casting conditions such as a width of
casting, a thickness of slab cast, a casting speed, sorts of
immersion nozzles and the like are varied in a wide range. A
frequency of electric current is represented with the ordinate. A
casting condition is represented with an effective braking
parameter E of the abscissa and an angle .alpha. formed by an axis
of an exit port of an immersion nozzle in the direction of the
molten steel poured and the horizontal line.
In case that the stream of the molten steel poured from the exit
port of the immersion nozzle goes out of the lower limit of the
linearly shifting magnetic field, i.e., the angle .alpha. is in the
range of 60.degree. to 35.degree. directed downwardly, the
effective braking parameter E is represented by the formula
E=(A.multidot.B.multidot.C)/{N.multidot.(W-D).multidot.S}. In case
that the stream of the molten steel poured from the exit port of
the immersion nozzle is in the range of the upper limit and the
lower limit of the linearly shifting magnetic field, i.e., the
angle .alpha. is in the range of over 25.degree. directed
downwardly and below 15.degree. inclusive, directed upwardly, the
effective braking parameter E is represented by the formula
E=4.multidot.B.multidot.C(cos .alpha.).sup.2
/{N.multidot.A.multidot.S}. In FIG. 13, the straight line(a)
represents a case where the angle .alpha. is in the range of from
60.degree. to 35.degree. directed downwardly, the straight line(b)
a case where the angle .alpha. is in the range of over 35.degree.
to 25.degree. directed downwardly, and a case where the angle
.alpha. is in the range of over 25.degree. directed downwardly and
below 15.degree. inclusive, directed upwardly.
When the effective braking parameter E has a comparative small
value of from 1 to 2, a width of a mold is comparatively small or a
casting speed is small. When E has a value of from 1 to 2, the
lower limit of a frequency of electric current which depresses the
wave of the molten steel surface is 0.8 Hz or less. The value of
the effective braking parameter is increased as the width of the
mold is or as the casting speed is increased. The lower limit of
the frequency of electric current for depressing the wave of the
molten steel surface shows a straight line rising right-wardly with
an increase of the value of the effective braking parameter.
However, the upper limit of the frequency of electric current
allowing the magnetic permeability to be lower is constant
irrespective of the width of the mold and the casting speed.
An example of the casting as shown in FIG. 12 is shown in FIG. 13.
Symbols , , .largecircle. and .quadrature. correspond to those of ,
, .largecircle. and .quadrature. shown in FIG. 12.
Symbol .largecircle. of FIG. 12 represents a case where the width
of casting is 1550 mm, the casting speed is 2.0 m/min, and the
angle of the axis of an exit port of an immersion nozzle relative
to the horizontal line is 45.degree. directed downwardly, but a
point of symbol .largecircle. in FIG. 13 is located below a
straight line of the lower limit of the frequency of electric
current shown by the angle .alpha. of 45.degree.. In line C
represented with symbol .largecircle. in FIG. 12, the wave of the
molten steel surface is accelerated when the value of electric
current is increased. This is because there have been produced some
portion of the stream of the molten steel poured from the immersion
nozzle which have undergone an electromagnetic braking force and
other portion thereof which have not. The wave of the molten steel
surface has thus been increased.
Symbol represents a case where the width of casting is 950 mm, the
casting speed is 1.6 m/min, the angle of the axis of an exit port
of an immersion nozzle relative to the horizontal line is
45.degree. directed downward, and the lower limit of the frequency
of electric current 0.43 Hz. The frequency of electric current was
1.0 Hz, which is substantially two times larger than the lower
limit of the frequency of electric current. Since the magnetic
field is generated with a frequency of electric current greater
than the lower limit of the frequency of electric current of 0.43,
the effect of braking the wave of the molten steel surface is
sufficiently produced.
In FIG. 13, a case is shown where the stream of the molten steel
poured from the exit port of the immersion nozzle has not yet gone
out of the range of the upper limit and the lower limit, i.e. the
angle .alpha. of the exit port of the immersion nozzle is in the
range of over 25.degree. directed downwardly and below 15.degree.
inclusive, directed upwardly. Symbol .circleincircle. shown in FIG.
13 is a case where the width of casting is 2100 mm, the thickness
of a slab cast is 250 mm, the casting speed is 2.0 m/min., and the
angle .alpha. of the exit port of the immersion nozzle is
15.degree. directed downward. The effective braking parameter E is
1.1, the frequency of electric current of lower limit 0.40 Hz. Even
the frequency of electric current being of the standard level of
the lower limit of 0.40 Hz is effective in depressing the wave of
the molten steel surface. Since this is in the range where the
product of the square of the magnetic flux and the frequency of
electric current is expected to be increased even if the frequency
of the electric current is further increased, the casting has been
carried out by the frequency of 1.2 which is 3 times as large as
the frequency of electric current of the lower limit. By this 1.2
Hz, the wave of the molten steel surface has been more effectively
depressed. Symbol .DELTA. shown in FIG. 13 is a case where the
width of casting is 700 mm, the thickness of a slab cast 250 mm,
the casting speed is 3.0 m/min. and 1.5 m/min., and the angle
.alpha. the exit port of the immersion nozzle 5.degree. directed
downward. The effective braking parameter E is 5.0 and 2.5, the
frequency of electric current of lower limit 1.30 Hz and 0.65 Hz.
In the case where the casting speed is 3.0 m/min., the frequency of
electric current is doubled to be 2.60 Hz, and in the case where
the casting speed is 3.0 m/min. the frequency of electric current
is doubled to be 1.30 Hz. In both cases, the wave of the molten
steel surface is effectively depressed.
In FIG. 14, a straight line showing the lower limit of the
frequency of electric current and a straight line showing the
frequency of electric current obtained by multiplying the lower
limit of the frequency of electric current by an integer are
represented when the angle .alpha. of the exit port of the
immersion nozzle is in the range of 60.degree. to 25.degree. both
directed downwardly. In FIG. 14, r=1 is for the standard frequency
of electric current of the lower limit, r=2 is for two times of the
standard frequency, and r=3 is for three times standard
frequency.
In the case of symbol , a frequency substantially two times larger
than the lower limit of the frequency of electric current is used.
Since the stream of the molten steel poured from the immersion
nozzle undergoes an electromagnetic braking force twice during its
passing through the area to which the linearly shifting magnetic
field is introduced, the wave of the molten steel surface is
depressed to such an extent as satisfied. In this way, the
selection of frequencies is not limited to the lower limit of the
frequency of electric current. The lower limit of the frequency of
electric current or more, or frequency two times or three times
larger than the lower limit of the frequency of electric current
can be used. However, unless the frequency of electric current is
below the upper limit of the frequency of electric current allowing
the permeability to be lowered, the effect of depressing the wave
of the molten steel surface cannot be produced.
As described above, according to the present invention, a wave of a
molten steel surface in a mold can be well depressed by driving the
magnetic field generator within a range of frequencies of electric
current, even when the casting speed is comparatively large and the
width of the mold is large. In consequence, the entanglement of
mold powder in the molten steel due to the wave of the molten steel
surface is prevented. Moreover, since a violent disturbance of the
molten steel, which is generated together with the wave of the
molten steel surface, is prevented, mold powder entangled in molten
steel and non-metallic inclusions in molten steel, which are
generated in a process of refining, are not prevented from rising
to the surface of molten steel in the mold, which facilitates the
removal of those inclusions from the molten steel in the mold.
* * * * *